Method of producing moving coil loud-speaker



Dec. 16, 1958 H. c. WILLSON 7 2,364,155

METHOD OF PRODUCING MOVING COIL LOUD-SPEAKER Filed Jan. 2. 1953 2 Sheets-Sheet l ,C d 3 X Inventor Attorney Dec. 16, 1958 wlLLsoN 2,864,155

METHOD OF PRODUCING MOVING COIL LOUD-SPEAKER Filed Jan. 2. 1953 2 Sheets-Sheet 2 United States Patent METHOD OF PRODUCING MOVING COIL LOUD-SPEAKER- Harry Claude Willson, Wolverhampton, England, assignor toReproducers and Amplifiers Limited, Wolverhampton, England' a British company Application January. 2', 1953,.Serial No. 329,449

3 Claims. (Cl. 29,--155.5)

This invention relates to moving coil loudspeakers and is of particular importance in connection with; such loudspeakers for use in television receivers.

In many types of modern television receiver, it is frequently necessary, for reasons of economy of space, to place the loudspeaker close, to the cathode ray tube, with its axis at right angles tothat of the tube. The two axes may or may not lie in the same horizontal plane, but in either case it is found that the stray magnetic field caused by the loudspeaker tends to displace and distort the image on the television screen.

Attempts have been made to minimise this objectionable stray field, which is along and adjacent to the axis of the loudspeaker, by'means of magnetic screening and by constructing the chassis of the loudspeaker of nonmagnetic material, but this has'not' proved entirely successful'. It has now been found that a relatively large proportion of the stray field, of the order of half, is caused by residual magnetism in the chassis and other parts ofthe. loudspeaker madeof magnetic material, this proportion of the stray field being produced during the normal process of magnetisation.

According to the present invention, the stray magnetic field along and adjacent to the axis of a moving coil loudspeaker of the enclosed permanent magnet type having a central magnet is reduced to within predetermined narrow limits on either side of zero by reversing the residual magnetism of such components so that the resultant external field along the axis of the loudspeaker due to these components is in opposition to that due to the magnet system itself and substantially cancels it out. In order to achieve this desired reversal, the loudspeaker, after magnetisation may be placed in a field of opposite polarity, but of considerably less strength than the normal magnetising field. This serves to reverse the residual magnetism in the chassis and while at the same time producing no appreciable reduction in the flux density in the working gap of the loudspeaker.

A method in accordance with the invention will now be described in more detail by way of example. In order to carry out the necessary adjustments in the neutralising of the stray field along its axis, a loudspeaker of the enclosed permanent magnet type having a central magnet is mounted in a jig with its axis at right angles to that of a cathode ray tube so that the rear of the magnet assembly of the loudspeaker is at a distance from the cathode ray tube corresponding to the distance to be used in practice. In the normal, fully-magnetised condition of the loudspeaker the image on the screen of the cathode ray tube will be deflected in one direction or the other in relation to its datum position, and will be returned to a position at or close to the datum position when the residual magnetism of the components of the loudspeaker is reversed.

The effect obtained is illustrated in the annexed drawings in which:

Figure 1 shows the components of a loudspeaker before treatment in accordance with the invention;

Figure 2 showsthese componentsiu the, assembled po: sition with an indication. of the relevant portion, of the stray field produced;

Figure 3 shows the displacement of the raster on. the screen of 'a cathode ray tube; and

Figures 4, 5 and 6 respectively show, corresponding views when the loudspeaker has been treated in accordance with the invention.

The loudspeaker shown in Figure 1 comprises. a slug 1 of permanent magnet material enclosed within a, soft iron pot 2 and provided with inner and outer pole pieces 3 and 4 respectively to leave an annular air. gapv 5 between them. As shown the slug hasa north pole at its upper end and a south polev at its, lower end which extends around the walls of the pot.

A chassis6. supports a cone 7 carrying a moving coil 8 working in they air gap 5. Under normal conditions of. magnetisation, the chassis will be magnetised in the same direction as the slug 1 and will have a north pole extending round its upper end and a south pole at its lower end. When in the assembled position of Figure 2, these. components will produce afield having a relevant portion of the general shape shown, i. e. one in which the lines of force leave the north pole of the magnet slug and pass in an approximately elliptical path round to the south pole, entering this south pole over an area distributed across thebottom of the pot, and hence producing a stray magnetic field along the axis of the magnet as shown.

A. cathode ray tube located in this stray field will have its raster displaced as shown in Figure 3 by an amount shown as x. Moreover the raster will be slightly tilted as shown so that each end deviates by a distance y from the mean position because one side of the tubeis closer to the loudspeaker than the other and hence lies in a stronger field.

After treatment in accordance with the invention, the residual magnetism of the chassis and other components is reversed as shown in Figure 4. Thus although the magnetism of the slug will not be affected in any Way and north and south poles will remain at the top and bottom respectively, the chassis will now have a south pole at the top and a north pole at the bottom. These produce a resultant field, having a relevant portion as shown in Figure 5. In this case, the majority of the stray lines of force leaving the north pole of the magnet flow to the south pole of the chassis. Similarly lines of force from the north pole of the chassis split up, a part of them flowing to the south pole of the chassis and the remainder flowing to the south pole of the permanent magnet. This remainder is a very small proportion of the original stray field and flows in at the bottom corners of the magnet slug 1, leaving the central portion along the axis of the magnet without any substantial field. Under these conditions, no deflection is produced on the raster of the cathode ray tube, and as shown in Figure 6, no distortion is produced.

The exact strength of the reverse field required in any particular case has to be determined experimentally, but in general it is found to be less than the value necessary to produce any appreciable change in the flux density in the working gap of the loudspeaker. In one particular example, it was found that a current of 1.6 amperes in the reverse magnetising coil was sufiicient to produce the required reversal while 2.5 amperes was the minimum current to produce any appreciable change in the flux density in the working gap. The reverse-field coil is shown diagrammatically at 10 in Figure 5, and the slugmagnetizing coil is represented at 11 in Figure 2. A single coil may be used for both purposes but it is preferred to use different coils because of the high magnetization required by coil 11. In practice, the current in the reverse field coil is increased step by step until, when the loudspeaker is placed in the jig, it is found that the image on the screen is returned to its datum position. Once the necessary value of current has been determined, similar loudspeakers can then be treated without further tests. In point of fact, a slight over-correction may be applied so that as the reverse residual magnetism of the loudspeaker slowly decreases owing to shock and ageing, the tendency is for the stray field to approach the zero value.

Although the invention has been described as being of primary application to loudspeakers for use in a television receiver, it is of course also usefulin other cases where a stray field may be objectionable, such as for example, the installation of loudspeakers in aircraft.

I claim: I l. The method of producing a moving coil loudspeaker of the substantially totally enclosed permanent magnet type having a central magnet and carrying a cone-supporting chassis formed of magnetic material, and wherein the stray magnetic field produced by said chassis along and adjacent the axis of the loudspeaker is to be reduced to a negligible value, said method comprising the steps of assembling the component parts of the loudspeaker with said central magnet substantially unmagnetized, subjecting said loudspeaker including said central magnet and said chassis to a strong magnetic field directed along the axis of the loudspeaker and of sufiicient strength to permanently magnetize said central magnet to the desired strength and to magnetize the said chassis in the same direction as said central magnet, removing said magnetic field, and establishing a magnetic field from a source external of said loudspeaker assembly and along the same axis as said strong field, but in the reverse direction, and of lower strength sufiicient only to reverse the residual magnetism in the said chassis and to leave a residual field in the opposite direction to that produced by said central magnet.

2. A method of producing a moving coil loudspeaker of the enclosed permanent magnet type having a central magnet located within an enclosing magnetic structure substantially totally enclosing the magnet and having other magnetic components mounted on said enclosing magnetic structure externally thereof, said externally mounted magnetic components having substantially less retentivity than said permanent magnet, and wherein the stray magnetic field produced by said magnetic components along and adjacent the loudspeaker axis is to be reduced to a negligible value, said method comprising the steps of assembling the parts of said loudspeaker with said central magnet substantially unmagnetized, placing said assembled loudspeaker with said components assembled thereon in a magnetic field of sufficient strength to permanently magnetize said central magnet and establishing a residual field in said external components, and placing said assembled loudspeaker in a magnetic field formed externally of said loudspeaker and of opposite polarity with respect to said first-mentioned magnetic field and of a strength just suflicient to reverse the residual magnetism of said external'component parts without substantially affecting the field of said central magnet.

3. A method according to claim 2, in which the reverse magnetic field is established at a strength such that the external field along the axis of the loudspeaker due to the reversed residual magnetism of the external parts is slightly greater than that due to the magnet system.

References Cited in the file of this patent UNITED STATES PATENTS 1,886,816 Jensen Nov. 8, 1932 1,941,476 Jensen Jan. 2, 1934 1,941,477 Jensen Jan. 2, 1934 2,020,211 Quam Nov. 5, 1935 2,272,766 Corson et a1. Feb. 10, 1942 2,666,980 Russell Jan. 26, 1954 2,790,116 Kirby Apr. 23, 1957 

